I got a promotional ticket for a free cruise to Helsinki on Silja Line and thought it would be an excellent use of a weekend. So I brought up the Silja web site, clicked on “Cruise” and started filling out the cruise booking form. This was divided into multiple pages, requiring input of lots of things and only on the last page did I get the chance to input a discount code. This made no difference to the price. Maybe the code was wrong. Instead I tried the serial number of the ticket. This was also accepted by the form, but didn't cause any change in the price either. Weird.
The next day I called the online booking helpdesk and explained my problem. The helpdesk person explained, in a tone suggesting he did this for nine out of ten calls, that I had to do an “online booking”
Well yes, that's what I did?
Push the button on the right marked “Online booking”.
On the right? Oh, that's a button?
Clicking the marked region brought up a form similar to the earlier one but with the difference that it had an extra field for entering a discount code at the bottom (visible by scrolling).
I leave it as an exercise for the readers to enumerate the ways in which this interface design can be improved.